In a hybrid-type shovel such as a hybrid-type hydraulic shovel, it is usual to assist an engine by driving an electric motor, which is referred to as an assist motor, by an electric power supplied from a battery (for example, refer to Patent Document 1). A permanent magnet motor, such as an IPM (Interior Permanent Magnet) motor, is used as an assist motor in many cases.
In the IPM motor, a rotor having a permanent magnet embedded therein is arranged inside a stator provided with coils. By supplying an electric current to the coils of the stator to generate a rotating magnetic field, the rotor is rotated by an action between the rotating magnetic field and a magnetic field generated by the permanent magnet of the rotor. Accordingly, the rotating force of the rotor, that is, an output of the IPM motor, is proportional to the magnetic field generated by the permanent magnet embedded in the rotor.
Although the permanent magnet embedded in the rotor is a strong magnet, the magnetic force may be reduced (degaussed) if it becomes a high-temperature, and the magnetic force may be attenuated gradually due to aging. If the magnetic force of the permanent magnet embedded in the rotor is reduced, the output of the IPM motor is reduced.